Evolution of the semiconductor manufacturing industry is placing greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions continue to shrink, yet the industry needs to decrease time for achieving high-yield, high-value production. Minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for a semiconductor manufacturer.
Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
Electron beams are used in a number of different applications during semiconductor manufacturing. For example, electron beams can be modulated and directed onto an electron-sensitive resist on a semiconductor wafer, mask, or other workpiece to generate an electron pattern on the workpiece. Electron beams also can be used to inspect a wafer by, for example, detecting electrons emerging or reflected from the wafer to detect defects, anomalies, or undesirable objects.
These inspection processes are used at various steps during a semiconductor manufacturing process to promote higher yield in the manufacturing process and, thus, higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary because even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Photocathodes have been used to generate electron beams. A single light beam incident on a photocathode system can generate a single electron beam with high brightness that is capable of delivering high electron current density. Single wavelengths used to generate an electron beam were not tailored to the energy bands of the photocathode material. Thus, the quantum efficiency (QE), emittance, energy spread, and heat dissipation are not optimized.
Alkali halide photocathodes such as CsI and CsBr have demonstrated photoemission from intraband states when illuminated with wavelengths much longer than their bandgap energy. So far, the illumination schemes to pump these photocathodes involve either short wavelengths with energies larger than the bandgap or longer wavelengths that first activate the color centers located at about 4.7 eV above the valence band. These schemes have been tried both in transmission and in reflection mode. For reflection mode, 257 nm and 266 nm beams have successfully activated the color centers and photogenerated electrons in vacuum. A 410 nm beam was not successful at activating defects and simultaneously transferring the electrons to vacuum.
Increased quantum efficiency has been demonstrated when alkali halide materials are in contact with an InxGa(1-x)N p-doped layer. The enhanced quantum efficiency of this structure is because the intraband states level aligns with the InGaN valence band (VB), which provides a pathway of the photogenerated electrons to vacuum.
However, previous material combinations were not designed specifically to overcome the tradeoff between quantum efficiency and emittance that limits the source brightness. Furthermore, dual wavelength excitation schemes or dual transmission/reflection excitation schemes were not used for brightness optimization of the different layers.
Therefore, improved photocathode designs and methods of operation are needed.